To obtain a grease capable of exhibiting a long life under high temperatures, oxidation inhibiting properties of the grease should be increased. For this purpose, the resistance to oxidation of the base oil, thickener, and additives has been improved in many conventional greases.
For example, various greases where the thickener comprises a diurea compound and the base oil comprises an alkyl diphenyl ether oil are proposed as a grease compositions for high-temperature and high-speed bearings, that is, for automotive electric and electronic equipment, engine auxiliary machinery and the like (as disclosed in Japanese Patent Unexamined. Publication (JP Kokai) Hei 03-028299, JP Kokai Hei 05-098280, JP Kokai Hei 06-017079, JP Kokai Hei 04-253796 and the like).
There is also proposed a grease composition aimed to reduce early seizing under conditions of high temperatures and high speeds by adding 10 mass % or more of ester oil to the base oil. The above-mentioned grease composition, however, is not necessarily provided with satisfactory performance.
There is no precedent for other additives than antioxidants being used in order to improve the oxidation inhibiting properties of the grease. The oxidation inhibiting effect of the antioxidant is inevitably determined by interaction of the antioxidant with the oil. In light of this, to demand oxidation stability means a request for the oil itself. The improvement is therefore limited so far as the kind of oil is limited. In consideration of the oxidation inhibiting mechanism, a drastic improvement in the oxidation inhibiting effect cannot be expected when the antioxidant is used alone. (“Sekiyu Seihin Tenkazai” (Additives for Petroleum Products), by Toshio SAKURAI, Saiwai Shobo Co., published May 15, 1973, pp. 379-380)
There are few novel products of base oil, thickener and antioxidant. It becomes difficult to make further improvement at the present stage.